The invention concerns generally the mechanical and electrical structure of a keypad or keyboard. Especially the invention concerns a keypad or keyboard where a layer of at least partially conductive domes is used to establish conductive connections in response to key presses.
Membrane keypads appear typically in portable electronic apparatuses such as mobile telephones, palmtop computers and personal digital assistants. The term membrane keypad refers to an array of pressable keys where significant portions of the electromechanical structure consist of layers that extend essentially throughout the whole coverage area of the keypad. FIG. 1a is a partial cutout exploded view which illustrates a known structure for such a keypad. The upper surface of a printed circuit board 101 comprises a number of contact areas where at least two conductive strips 102 and 103 come close to each other. Supported over the printed circuit board 101 and separated from it by a perforated insulation layer 104 is an array of conductive domes 105 so that each conductive dome 105 lies directly above the conductive strips 102 and 103. A key mat 106 is located on top of the conductive domes. It is made of an elastic material and has a bulging protrusion 107 on top of each conductive dome. A perforated outer cover 108 comes on top of the aforementioned structural components so that each bulging protrusion 107 of the keymat protrudes slightly out through the holes in the outer cover.
The key mat 106 is made of translucent silicon rubber or some other suitable material that conducts light reasonably well so that a number of surface-mounted light emitting diodes (SM-LEDs) 109 can be used for illuminating. The SM-LEDs 109 are soldered to contact pads 110 and 111 so that in the assembled configuration their principal direction of emission is through an edge of the key mat 106 into the substance thereof. Light escaping through the upper surfaces of the bulging protrusions 107 provides the visual illuminating effect seen by the user. The upper surfaces of the bulging protrusions 107 may be partly covered with an opaque layer so that light can only escape through an opening which may have the outline of a character or a group of characters. FIG. 1b shows the same structure in assembled configuration.
FIG. 1c shows a simple circuit diagram of the illumination arrangement; the keypad functionality is not shown in FIG. 1c. The SM-LEDs 109 used for illumination constitute a serially coupled chain, the cathode side end of which is coupled to ground potential. The anode side end of the chain is coupled to a microprocessor 120 which comprises a controllable switch 121 between the anode side end of the SM-LED chain and a positive supply voltage Vcc. The microprocessor 120 comprises also means 122 for detecting the need for illumination of the keypad. The block 122 shown in FIG. 1c is typically a software process which is arranged to monitor the functional state of the keypad-controlled device in question and to generate an output pulse that closes the switch 121 whenever the functional state suggests that illumination of the keypad would be advantageous.
The illuminating arrangement of FIGS. 1a, 1b and 1c only enables the illumination of the whole keypad at a time. It would naturally be possible to group the SM-LEDs 109 into two or more subchains which could be coupled to a positive voltage independently of each other. However, even in such a further developed arrangement it is difficult to control the propagation of light in the substance of the key mat 106 so that only a part of the keypad would appear to be illuminated. Increasing the number of subchains would also directly increase the required I/O (input/output) pin count of the microprocessor 120, which is not a preferable trend in development. A further drawback of the arrangement of FIGS. 1a, 1b and 1c is that the SM-LEDs 109 take a certain space (in the order of 0.6 mm at minimum) in the vertical direction. Also other parts of the keypad arrangement have their respective non-zero thicknesses, so that altogether the illuminated keypad arrangement becomes rather thick.
It is an object of the present invention to provide a keypad arrangement the thickness of which is relatively small. It is an additional object of the invention to provide a thin illuminated keypad arrangement. It is a further object of the invention to provide a keypad illuminating arrangement which can be reconfigured for different kind of illumination effects with minimal effort.
The objects of the invention are achieved by inverting the domes in a keypad arrangement from their conventional upwards bulging orientation, so that the domes bulge towards the mechanical support structure of the keypad arrangement rather than towards the tactile surfaces of the keys. Certain other objects of the invention are achieved by using light sources made of layered foil structures, such as polymer based organic LEDs, for illuminating keys or key groups. Certain objects of the invention may even be achieved though other light sources, such as SM-LEDs, are used if the mechanical structure of the keypad is correctly designed.
The invention concerns a keypad for a portable electronic device. The characteristic features of a keypad according to the invention are recited in the independent patent claim directed to a keypad.
Conventionally it has been thought that the at least partially conductive domes that constitute the dome sheet or dome layer in a membrane keypad must bulge xe2x80x9cupwardsxe2x80x9d, that is, towards the outer surface at which the user sees and operates the keypad. In a conventional arrangement the action of pressing a key causes a point-like force to be exerted downwards against the center of a dome. According to the present invention it is perfectly possible and even advantageous to make the domes bulge downwards instead. Here xe2x80x9cdownwardsxe2x80x9d means the direction away from said surface at which the user sees and operates the keypad.
The invention does not place specific requirements to the light sources that are used to illuminate the keypad. There may even be no illumination at all. However, the mechanical structure according to the invention tends to make certain illumination arrangements more advantageous than others.
Organic LEDs, also known as OLEDs, are light-emitting semiconductor devices that consist of a stack of very thin material layers. A simple OLED structure consists of a transparent substrate, an anode electrode layer, a polymeric recombination layer and a cathode electrode layer on top of each other. In a keypad arrangement according to the invention, one or more OLEDs can be associated with individual keys or key groups so that they take on the role of the known surface-mounted diodes in illuminating the keypad. The OLEDs are integrated into the keypad structure so that one of the layers that would exist in the keypad structure also for other reasons also acts as the substrate layer for the OLEDs. An alternative solution is to manufacture complete OLEDs and to attach them to the other parts of the keypad arrangement for example by glueing or soldering them to a surface or embedding them into an injection moulded piece of material.
An OLED or comparable semiconductor light source used for keypad illumination can even be grouped together with a solid-state switching element such as an organic field-effect transistor, also known as OFET. The advantage gained through such grouping comes from the fact that a single pair of supply voltage lines can be used for a number of light sources, and still at least one of the light sources (the one grouped together with a switching element) can be individually switched on or off by using a simple, low-power control signal. A converter unit is typically required for converting the illumination controlling commands issued by a microprocessor into switching signals for the individual light sourcexe2x80x94switching element pairs.
An illuminated keypad according to the invention typically consists of layers so that a dome layer or dome sheet layer is not on top of the layer comprising the contact points but under it. Combining such an order of layers to the use of OLEDs as light sources for illumination results in a solution where the light sources may be located very close to the visible surface of the illuminated keys. The advantage gained therethrough comes from the fact that when the emitted light has only a very short passage to travel within a transparent or translucent substance before being emitted into air and towards the eyes of a user, only a fraction of that electric power must be used for illumination which was used to drive the light sources in conventional solutions, without compromising the observable intensity of light. A naturally occurring alternative viewpoint is that with the same amount of electric power used for illumination as in conventional solutions a far brighter lighting effect can be obtained.